There are numerous pneumatic drills employed for rock drilling. These drills having various components and widely differing configurations. U.S. Pat. No. 3,858,666 teaches a drill having a percussion unit with a single stem piston. The stem has air passageways therein to control the flow of compressed air which is used to reciprocate the piston. Drills so valved are known as valveless drills.
U.S. Pat. No. 4,243,110 discloses another valveless drill and employs a percussion unit. This drill employs a two stem piston. Both of the above drills have an in-line percussion unit, air motor, and striking bar. However since the piston is double valved the inherent length of the drill is longer than that of single stem drills of similar cross-section and stroke.
U.S. Pat. No. 4,340,121 is a valveless drill of the double stem variety which is forshortened compared to the drill of the '110 Patent. The forshortening being accomplished by offsetting the air motor percussion unit. The reduction in length is obtained at the expense of the overall diameter of the drill. One of the disadvantages of the double stem drills is that they generally are heavier than single stem drills in part because they are not as compact.
The pistons which are employed in percussion units are frequently defined as thick or thin head pistons depending on the ratio of the thickness of the piston head to its diameter. The pistons employed in the percussion unit for drills classically are designed as thick head pistons as illustrated in the '666 and '110 Parents. The thick head pistons distribute any load imparted by the piston to the cylinder permitting small clearances between the cylinder and the piston. The disadvantage of this design is it requires massive pistons which absorb large quantities of energy in the non-productive back stroke of the piston.
U.S. Pat. No. 3,991,835 eliminates the requirement of a thick piston by providing a continuous open space between the piston and the cylinder in which it reciprocates. The patent defines a piston as a thin head piston when the ratio of the thickness to diameter is less than about 0.17 to 1. Support for the piston is provided by a stem section of the piston which slidably engages a bearing surface to direct the motion of the piston.
The lighter piston reduces the energy expended in the back stroke of the piston as well as allowing an increase in the frequency of operation resulting from the reduction in the piston mass.
There is commercially available a rock drill produced by Sullivan Industries which has an offset air motor and incorporates many features taught in the '121 Patent. The uses a thin head having its thickness to diameter less than 0.22 thus being less than the minimum value for thick pistons as taught in the '835 Patent. The ratio is similar in appearance to the piston illustrated in the '121 Patent. A continuous open space is provided between the piston and the cylinder as taught in the '835 Patent. The piston is supported by distributors which slidably engage the piston stems. This drill has met with commercial success and provides rapid cutting rates. However these drilling rates are obtained at the expense of air consumption. Thus there is a need for a more air efficient pneumatic drill with enhanced performance.